Light metal piston having heat expansion control inserts

ABSTRACT

The skirt of a light metal piston is divided into a short inner portion and a long outer portion between which heat expansion controlling inserts are embedded. The inner and outer skirt portions are separated from each other to form cavities midway between the wristpin bosses and the ends of the inserts are spaced by the cavities. The cavities and inserts extend up to the lowermost piston ring groove.

United States Patent [72] Inventor Rudolf Mlkl Deidesheimer Str. 27, 7 Stuttgart-Wei] im Dori, Germany [211 App]. No. 855,138 [22] Filed Sept. 4, 1969 [45] Patented Aug. 10, 1971 [32] Priority Oct. 18, 1968 [33] Germany [31] P18 03 832.9

[54] LIGHT METAL PISTON HAVING HEAT EXPANSION CONTROL INSERTS 1 Claim, 2 Drawing Figs.

[52] US. Cl 92/228 [51] Fl6j H06 [50] Field 01 Search 92/214, 225, 226, 227, 228, 229, 230, 233, 237, 238

[56] Rderences Cited UNITED STATES PATENTS 1,678,768 7/1928 Davis 92/214 Primary Examiner-Martin P. Schwadron Assistant Examiner-Irwin C. Cohen Attorneys-Francis D. Stephens and Hugo Huettig, Jr.

ABSTRACT: The skirt of a light metal piston is divided into a short inner portion and a long outer portion between which heat expansion controlling inserts are embedded. The inner and outer skirt portions are separated from each other to form cavities midway between the wristpin bosses and the ends of the inserts are spaced by the cavities. The cavities and inserts extend up to the lowermost piston ring groove.

LIGHT METAL PISTON HAVING HEAT EX PANSION CONTROL INSERTS This invention relates to a light metal piston and heat expansion inserts joined to the piston'skirt.

In particular, this invention relates to a light metal piston having a pistonhead joined to a piston skirt and having wristpin bosses which are connected to the skirt and by means of ribs to the piston head. This invention is of the type in which the radial expansion of the skirt is controlled by inserts extending substantially parallel to the curvature of the skirt and made of a material, such as sheet metal, having a smaller coefficientof heat expansion. The sheet metal inserts extend axially of the piston up to the bottom of'the lowermost piston ring groove, and the inserts extend across'and outwardly from the wristpin bosses. The upper end of the skirt is at least partially spaced from the piston ring zone.

This type of piston is disclosed in German Pat; No. 1,140,023. The piston has transverse slots partially separating the skirt from the piston ring zone, which slots extend'into the area in which the supporting ribs connect the wristpin bosses with the pistonhead, while on the other hand, the heat expansion control inserts extend across the remaining circumference of the piston. 7

Even though the supporting ribs ofsuch pistons are inclined at a suitable angle with regard to the longitudinal piston axis and thus extend over a wider than usual width, the resultis in that around the piston circumference are relatively narrow inserts of very little effect and relatively wide transverse slots which unfortunately impair the piston stability.

The object of this invention is to produce a piston of the above-described type in which a greater stability is obtained and at the same time a more effective control of the radial heat expansion of the skirt is possible.

In general, these objects are obtained in that the expansioncontrolling inserts extend circumferentially beyond the area of the supporting ribs and, secondly, between the inserts and the interior of the piston are light metal inner skirt portions which are directly connected to the piston ring zone of the pistonhead and which are in the form of a completely enclosed ring, and further that these inner skirt portions are separated from the outer skirt portions midway between the piston ring bosses to form axially extending cavities which further extend into the area of the lowermost piston ring groove. The length of the cavities corresponds to the spaces between the ends of the respective inserts which lie opposite one another.

This combination of features makes possible the use of heat expansion controlling inserts independent of the position of the supporting ribs because the inserts extend around a considerable portion of the circumference of the skirt and thus have a correspondingly large heat-controlling effect. Also, it makes possible that very short openings in the skirt can be used so that the stability of the piston is increased and the piston can be operated under greatst'ress.

The means by which the objects of this invention are obtained are described more fully with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view through the piston taken on the line H of FIG. 2; and

FIG. 2 is a cross-sectional view taken on the line ll-Il of FIG. 1.

As shown in the drawings, the piston has a pistonhead I in cluding a piston ring zone 2 and a skirt'3.'Wristpin bosses 4 are positioned in the piston skirt and joined to the pistonhead by means of supporting ribs 5. Sheet metal inserts 6 are cast into the piston for controlling the radial heat expansion ofthe skirt. These inserts extend substantially parallel to the curvature of the skirt and in the direction of the vertical axis of the piston up to the bottom of the lowermost piston ring groove 7 in the piston ring zone and which is nearest the skirt. In a circumferential direction, the inserts extend outwardly on both sides of the wristpin bosses to symmetrical planes at right angles to the axes of the wristpin bosses or, in other words, extend to points between the wristpin bosses in the skirt 3 and have their ends spaced by a'di'stance a.

The inserts 6 divide the piston' skirt into an outer skirt portion 31 and an inner skirt portion 32, the outer skirt portion 31 being separated from the pistonhead by the piston ring groove 7 and the inner skirt portion 32 being joined directly to the piston ring zone 2. Inner skirt portion 32 does not extend as far into the interior of the skirt 3 as does the insert 6. Midway between the wristpin bosses, the inner and outer skirt portions are separated to form cavities 8 7 extending in the axial direction of the piston, which cavities are separated over a distance a which corresponds to the space between the ends of the inserts and which cavities extend upwardly to the piston ring groove 7. Because ofthis, openings are formed at the top of the cavities 8 andat the level of the bottom of piston ring groove 7 which have a shape substantially similar to transverse slots but which, however, in contrast to the heretofore-used transverse slots end quite a distance from the supporting ribs 5.

The outer skirt portion 31 by reason of this construction is separated across the entire circumference of the skirt from the pistonhead, that is, from the parts that come in contact with the interior of the piston that have not been shown in the drawings, and skirt portion 31' is therefor not subjected to of the inner skirt portion 32.

During normal temperature, the outer skirt portion 31 has the conventional oval cross section which means that its diameter inthe direction of the axis of the wristpin bosses is slightly smaller in the order of a few tenths of a millimeter than is its diameter in the direction which is at right angles thereto. When the oute'rskirt portion 31 is heated during use, it ex ponds to be close to the shape of a circle because of the effect of the inserts 6. This effect is because of the fact that the inserts and the outer skirt portion 3I form bimetallic elements which are under shrinkage tension and which when being heated during use-undergo a reduction in the shrinking tension and tend to change their curvature in the sense of bending out to enlarge their radius of curvature. By reason of this, the original oval shape of the skirt becomes closer to the shape of a circle. Heat expansion therefor preferably takes place in the direction of the axis of the piston ring bosses while the outer skirt portions between the piston ring bosses are subject to a much smaller radial heat expansion. This effect can be influenced in any desired manner by a corresponding dimensioning of the insert 6. It is hardly influenced by the outer skirt portion 31 which lies between the ends of the insert 6 since they are flexible and have a thin wall because of the cavities 8. The effect is also not influenced by the inner skirt portion 32 becausethis, as already mentioned, is separated from the insert 6 and/or from the outer skirt portion 31 by means ofa disappearing slit and/or the cavities 8.

On the other hand, the inner skirt portion 32 which is joined directly to the annular piston ring zone 2 affects a considerable reinforcement of the top part of the skirt and makes it very resistant to the pressures from the pistonhead I, even though the supporting ribs 5 are only slightly inclined to the longitudinal axis of the piston and are therefor advantageously subjected only to pressures but not to bending stresses.

' Having now described the means by which the objects of the invention are obtained,

I claim:

1. A light metal piston comprising a pistonhead, a piston ring zone including a plurality of piston ring receiving grooves, an outer annular skirt extending downwardly from the lower surface of the lowermost piston ring groove, wristpin bosses positioned in said skirt and supporting ribs joining said bosses to said head, said outer skirt extending downwardly from and most edge of said inner skirt, each said insert extending in either direction from a said wristpin boss, the ends of said inserts being spaced from the ends of the other insert at points equidistant on each side from said wristpin bosses, and said outer and inner skirts being separated from each other radially over the distance between the ends of said inserts. 

1. A light metal piston comprising a pistonhead, a piston ring zone including a plurality of piston ring receiving grooves, an outer annular skirt extending downwardly from the lower surface of the lowermost piston ring groove, wristpin bosses positioned in said skirt and supporting ribs joining said bosses to said head, said outer skirt extending downwardly from and separated from said pistonhead by said lowermost piston ring groove, an inner annular skirt substantially shorter than said outer skirt forming a downward extension of the piston ring zone, and two inserts of metal having a lower coefficient of heat expansion than the light metal of said piston cast between said outer and inner skirts and extending from the level of the bottom of the lowermost piston ring groove downwardly to a position exposed to the inside of said piston below the lowermost edge of said inner skirt, each said insert extending in either direction from a said wristpin boss, the ends of said inserts being spaced from the ends of the other insert at points equidistant on each side from said wristpin bosses, and said outer and inner skirts being separated from each other radially over the distance between the ends of said inserts. 